An ultra-high-molecular-weight polyethylene porous membrane is used as a lithium ion battery separator or the like, and the pore-forming treatment is performed by volatilizing and removing the pre-impregnated organic solvent (Japanese Patent Application Laid-Open (JP-A) No. 2004-182763). Due to this organic solvent, there are concerns regarding the health problems of the workers in the production field, and there are problems regarding environmental load caused by discharging the organic solvent into the atmosphere. Therefore, a low-environmental-loading process for producing an ultra-high-molecular-weight polyethylene porous membrane without using such an organic solvent is required.
In addition, it is reported that pores are formed by stretching (JP-A No. 2009-249480), and the present inventors have also filed the application which discloses that pores are formed by stretching (International Application No. PCT/JP2010/053543). In the methods of the related art, pores are formed by stretching at a temperature of a melting point of polyethylene or lower and there is room for improvement in pore-forming efficiency.
For use in the above-described lithium ion battery separator, the thinner the membrane thickness (film thickness), the higher the output because more battery cells can be stacked. Therefore, a reduction in the thickness of an ultra-high-molecular-weight polyethylene film is desired. When an organic solvent is used, for example, JP-A No. 2004-182763 discloses that an ultra-high-molecular-weight polyethylene film is produced through kneading and extrusion steps. However, when an organic solvent is not used, it is difficult to stably mold a film due to a high melt viscosity of an ultra-high-molecular-weight polyethylene. Therefore, as a method for producing an ultra-high-molecular-weight polyethylene film without using an organic solvent, a skiving method which molds powder of ultra-high-molecular-weight polyethylene into a block shape in advance and skives it into a thin-layer film is industrially used (for example, JP-A No. 4-91926). However, the thickness of a film obtained in this method is greater than or equal to 150 μm, and thus it is difficult to obtain a thin-layer film in this method.
In International Application PCT/JP2010/053543, the present inventors disclose that polyethylene is molded, stretched, and shrunk to obtain a film without using any organic solvent, and that pores are formed in this film to form a porous membrane, and state that roll-molding or press-molding is applicable to the molding of polyethylene. In addition, JP-A No. 2003-165155 discloses that roll-molding can be applied to the molding of polyethylene without using any organic solvent. However, there is room for improvement for providing a thin-layer film having high strength and a superior gas barrier property or a porous membrane having superior ion permeability made from an ultra-high-molecular-weight polyethylene.